1. Technical Field
This application relates to a method of orbital milling with an orbital end mill. This application further relates to an orbital end mill and a cutting bit for an orbital end mill.
2. Background Information
End-mills are widely used in milling operations due to their versatile range of application and due to the moderate first cost of the tool. End-mills are often of cylindrical shape, and are available up to about 80 mm diameter. Many end mills have flat ends, however other shapes such as conical and rounded ends are also used. An end-mill typically has 2 to 10 teeth, depending on diameter, size and whether configured for rough cutting or finishing. Teeth are usually of spiral shape, but can be straight parallel to the axis. Material of construction is high speed steel, solid carbide, cermets or ceramic, or combinations thereof.
In the following text the words “end mill” refer to a cutter made of steel or of hard ceramic materials or combinations thereof, whether the cutter is plated with a hard coating or not. Also, the term “milling machine” as used in the present text is to be interpreted as also including other suitable machine tools, such as lathes, borers and heavy duty drilling machines.
An end mill normally held in a milling machine will, when fed into a work piece, produce a hole or recess of a diameter corresponding to the diameter of the cutting teeth of said end mill. However, where desired, it is possible to machine a hole of a size larger than the diameter of the end mill by mounting the end mill on a tool holder in a manner where the axis of the milling machine spindle and the axis of the end mill are spaced apart and parallel. This fixed orbital circle is, however, of limited use. Using a modern CNC milling machine, the end mill can be mounted normally and variable orbital movement is available, as needed, for example, for the machining of tapered holes. This orbital arrangement has been found to be particularly useful for the machining of blind holes and for the machining of female screw threads.
To further explain, orbital milling, which may sometimes be referred to as planetary milling, involves the movement of a milling head or bit along a circular or substantially circular path, or possibly other paths. Specifically, the central rotational axis of the milling head or bit revolves about and a distance away from another central axis, such as the central axis of a machine tool or the central axis of the hole being machined. Orbital milling permits the milling of a hole having a size or diameter that is greater than the greatest diameter of the milling or cutting head, often approximately double the size of the milling head. In operation, the milling head rotates about its central rotational axis while the entire milling head is moved, such as by a CNC tool holder, in an orbiting or revolving manner along a circular or substantially circular path, or possibly other paths, about another axis, such as the center axis of the hole. Often the direction of rotation of the milling head is opposite the direction of revolution, i.e. the milling head rotates in a clockwise direction while it is moved along a path in a counter-clockwise direction. Orbital milling not only permits the milling of larger holes with a smaller milling head, the increased space between the walls of the hole being cut and the perimeter cutting edges or surfaces or teeth of the milling head also promotes good chip flow and removal of chips and thus decreases the chances for jamming or even sticking of the milling head in the hole.
As is known, the tool cutting angles for rough machining requiring maximum rates of metal removal are different from the optimum tooth shape for finish machining where small amounts of metal are removed but a good surface finish is required. Obviously, it is possible to change the end mill on completion of rough machining and again exchange the end mill to machine the next component, or to rough machine a batch which is later finish machined. Neither option is desirable, as much time is lost either on tool changing or on work piece reclamping. For this reason double-purpose tools have been developed where an end mill or other tool carries at least one tooth for rough machining and a second tooth for finishing.
In U.S. Pat. No. 5,727,910, Leeb discloses a cutting tool including an insert having a plurality of cutting edges of inwardly directed V shaped geometry. The insert has inner and outer flank cutting edges with rounded corners. The cutting edge is divided into by a chisel edge into roughing and finishing cutting portions.
Two known problems, with regard to screw-on inserts, are that high precision in positioning the insert is required to avoid over-size holes, and the screw holding the insert has a tendency to loosen due to vibration of the tool when in use. Yet a further problem regarding tools using two inserts is that high precision is indispensable in the settings of the two inserts which need to be identical. Due to the size of the insert, which must be large enough to allow setting and clamping, small and medium size holes can not be machined.
A somewhat similar insert is described by Aström et al. in U.S. Pat. No. 6,193,446 B1. On side surfaces, a clearance surface formed on a protruding portion which, via a step clearance, extends into a secondary helically twisted clearance surface, the chip angle of which increases with increasing cutting depth. The insert includes a chip breaker.
Wardell, in U.S. Pat. No. 6,439,811 B1, claims an end mill wherein at least one flute defines a low angle cutting surface while a second flute has a high angle cutting surface. The two flutes intersect to form a compound cutting surface.
Kuroda et al. disclose a flat-bottom end mill with rounded corners in U.S. Pat. No. 6,846,135 B2. The corner configuration described is claimed to improve resistance to chipping and fracture of the end mill.